Eu ions are present in divalent and trivalent states, which respectively act as activators for phosphors. In particular, the divalent Eu2+ provides efficient phosphors because the divalent Eu2+ can efficiently absorb excitation energy by an absorption band of 4f-5d acceptable transition and can emit light. Phosphors having a host material of orthosilicate; α- and β-sialons; M2Si5N8 or MAlSiN3 in which M is an alkaline earth element; or like, activated with Eu2+, are efficiently excited by blue light, and provide fluorescence emission of green to red color with high efficiency, so that the phosphors are widely used as phosphors for white light emitting diodes (white LEDs).
For the Eu present in the phosphor, Eu3+ often coexists in addition to Eu2+ of interest. Therefore, to obtain high fluorescence properties, it is desirable to increase the ratio of Eu2+ present in the phosphor. The ratio of Eu2+ and Eu3+ can be quantified, such as by measuring a XANES spectrum of a Eu-L3 absorption edge. The XANES is an abbreviation of X-ray Absorption Near Edge Structure and is one spectroscopic method of X-ray absorption fine structure (XAFS) measurement methods. Strong absorption peak energy appearing in an L3 absorption edge XANES spectrum of a rare earth element is known to be determined by the valence of the rare earth element. In the case of Eu, peaks of Eu2+ and Eu3+ appear near 6970 eV and near 6980 eV, respectively, so that quantification can be performed by separating these two peaks.
For a phosphor that might contain Eu having various valences, the ratio of Eu2+ and Eu3+ has been calculated by the XAFS measurement method and the correlation between the fluorescence properties and the Eu2+/Eu3+ ratio has been discussed. For example, a β-sialon phosphor activated with Eu has been shown to have Eu2+/(Eu2++Eu3+) of 0.8 or more, and increased luminance (Patent Document 1).
The β-sialon phosphor activated with Eu has not only improved heat resistance and durability, and less change in luminance due to temperature rising, but also is excited with light having a wide wavelength from ultraviolet to blue light and emits green light having a peak in a wavelength range of 520 to 550 nm. Therefore, for the β-sialon phosphor activated with Eu, practical use has proceeded as a phosphor useful for the white LEDs. As stated above, the increased ratio Eu2+/(Eu2++Eu3+) improves the luminance of the β-sialon phosphor, but there is a strong need for further higher luminance. However, there has been a situation where it has been difficult to further increase the luminance of the β-sialon phosphor by simply increasing the Eu2+/(Eu2++Eu3+).